While all the determinants of tropism of HIV replication are still being elucidated, the major determinants reside in the envelope of the virus and are present in both the extracellular component (gp120) and the transmembrane component (gp41). However, other genes (gag, nef, vpr, vif) and cis-acting elements (the long terminal repeat, LTR) can also modulate replication in different cell types. Most primary isolates, particularly those of the NSI (non-syncytium inducing) class, fail to replicate in established CD4-positive cell lines, although peripheral blood mononuclear cells (PBMC) are almost always permissive even for NSI viruses. Viruses can frequently adapt to grow in cell lines, and such viruses have altered phenotypes. We have shown that the ELI strain of HIV-1 adapts to replicate in H9 and U937 cells, and the changes that confer this enhanced infectivity and expanded host range to this virus are in both gp120 and gp41. In particular, changes were found in the CD4-binding region of gp120 and the fusogenic region of gp41, thus identifying these regions as important determinants for tropism in cell lines. Concomitant with the increased infectivity was an increased ability of the virion to bind to CD4, although CD4 binding to the variant soluble gp120 was not affected. These results revealed that alterations in either gp120 or gp41 in the context of the virion can affect both the capacity of the envelope to interact with CD4 as well as infectivity, thus indicating the importance of these regions to the functional association of the two envelope components. To determine whether this result is general or specific to this particular strain of HIV-1, mutations were introduced at the corresponding position in the fusogenic region of the LAI and MAL strains of HIV-1. The aim was to obtain a virus with reduced replicative capacity and then by passage to obtain revertants that had restored the infectivity. The location of the changes in the revertants should identify interacting regions of gp41 or 120. The Leu of LAI was changed to Met and Val, and the Met of MAL was converted to Val. Substitution of the Met with a Val eliminated the infectivity of this virus. So far no revertants have been found. In LAI, substitution of Met for Leu had little perceptible effect on virus infectivity. However, replacing the Leu with Val resulted in a virus with lower infectivity. This latter virus was found to adapt to grow on CEM and H9 cells, and current work is engaged in determining the nucleotide changes that account for the increased infectivity of this virus.